The present invention relates to a field of energy, and particularly, to a field of battery material development and portable small-sized electronic equipment.
As electronic equipment has been more compact, a higher energy density has been demanded for a battery. In order to respond to such needs, a variety of nonaqueous electrolyte batteries such as a lithium battery have been proposed and put into practical use. However, particularly in the case of a secondary battery, a battery using lithium metal as a negative electrode has problems as follows:
(1) The battery requires 5 to 10 hours for charging and is inferior in quick charge.
(2) Cyclic lifetime is short.
These problems all result from lithium metal. It is understood that the causes are a change in the form of lithium caused by frequent charging and discharging, the form of dendrite lithium, non-reversible change of lithium and so on. Hence, as a method for solving the above problems, a carbonaceous material has been proposed as a negative electrode. This method uses a carbon intercalation compound of lithium that is electrochemically formed with ease. For example, when charging is performed in a nonaqueous electrolyte solution while using carbon as a negative electrode, lithium contained in a positive electrode is electrochemically doped between layers made of negative electrode carbon. And then, lithium-doped carbon acts as a lithium electrode, and lithium is de-doped while discharging between carbon layers and returns to the positive electrode.
JP 2513418 (B2) disclosed a black mix for a battery electrode contains as positive electrode active material (such as a manganese dioxide or lithium transition metal oxide), and as a positive electrode electroconductivity giving agent, a carbonaceous material containing carbon nanotube or carbonaceous material containing carbon nanotube including metal ions is added to this black mix.
JP 2526789 (B2) disclosed secondary battery having positive electrode, separator, nonaqueous electrolyte solution and negative electrode made of carbon material, characterized in that carbon material containing carbon nano-tube is used for active material of negative electrode.
In this battery, a carbon rod (having the diameter of 20 mm in the helium atmosphere of 500 Torr) is used as a negative electrode and a carbon rod of 10 mm is used as a positive electrode. A DC discharge is performed to obtain a carbon nano-tube containing carbonic material. The content is set to about 60%, the contained powder of 0.9 (g) and Teflon powder of 0.01 (g) are kneaded in an agate mortar to mold a sheet of 3 mm, and it is punched into a disk shape as a negative electrode active material 1. A lithium foil is used for a positive electrode active material 3 and a reference electrode 6. LiPF6 is solved to the concentration of 1 mol/l in a mixed solvent of ethylene carbonate and diethyl carbonate mixed at the volume ratio of 50%/50% for use as an electrolyte. A polypropylene porous film of 25 xcexcm is used for a separator 5. A stable characteristic can be obtained.
JP unexamined laid-open 10-125321 (A) disclosed a carbon material for negative electrode of battery, characterized in that said material is constituted in monolayer carbon nano-tubes with open ends those are aggregated in same axis.
In the battery using this material, a graphite rod containing 50% of nickel powder is used as a positive electrode, while a pure graphite rod is used as a negative electrode. 200-ampere DC discharge is caused across both the electrodes in a 400-Torr helium atmosphere, thereby forming a deposit on the negative electrode. This deposit is crushed and held in the air at 750xc2x0 C. for 30 minutes, and then cooled to room temperature. Thereafter, the crushed material is mixed with 1 mol of a nitrate water solution, and held at 140xc2x0 C. for 5 hours. A monolayer nano-tube has an overall tubular shape and has a single layer with one of ends 2a and 2b kept open. Also, the tube needs to contain lithium equivalent to charge and discharge capacity equal to or above 250 mAh per gram of negative electrode carbonaceous material. In the embodiment, the discharge capacity of 520 mAh/g and the discharge capacity of 410 mAh/g are respectively obtained.
JP unexamined laid-open 11-329414 disclosed an electrode comprising:
conductive matrix comprising disulfide groups, the Sxe2x80x94S bonds of said disulfide groups are splitted by
electrochemical reduction, and restored by electrochemical oxidation;
plural of carbon nano-tubes dispersed in said conductive matrix, and substantially disentangled.
In this electrode, an average diameter of carbon tubes is set to 3.5-200 nm, preferably, 5-30 nm, and the average length is set to at least 5 times or more the diameter, preferably, 100-10,000 times the diameter. The aspect ratio of carbon nano-tube is set to more than 5, desirably larger than 100, and more preferably, more than 1,000. The electrode has a structure substantially containing no carbon nano-tube aggregate. The electrode contains 0.5-6 wt. %, preferably 1-4 wt. % of carbon nano-tubes on the basis of the total of the conductive matrix and the carbon tubes.
At this moment, a charge quantity (mAh/g) per unit weight of a carbonaceous material is determined by a dope quantity of lithium. Thus, as for such a negative electrode, it is preferable to set a dope quantity of lithium as large as possible.
Further, regarding a battery material in which alkaline metal such as lithium is doped, when a dope quantity of alkaline metal increases, saturation occurs on a total quantity of charge transfer from alkaline metal to a carbonaceous material. In the case of a graphite structure, theoretically, an upper limit is a ratio of one alkaline metallic atom to six carbon atoms.
Even when more alkaline doping is performed, a quantity of charge injection does not increase. Thus, the above upper limit is a limit of a charge quantity per unit weight of a carbonaceous material. Moreover, due to chemical reaction of alkaline metal and surrounding materials constituting the battery, degradation of a battery material is inevitable after extended hours of use.
Therefore, the object of the present invention is to achieve battery materials which are reduced in mass relative to a charging capacity, increases in power supply, and is free from deterioration when used for a long time.
In order to solve the above-mentioned problems, in a battery described in claim 1 of the present invention, a single-layer carbon nanotube having a small diameter is placed in a single-layer carbon nanotube having a large diameter such that the nanotubes are insulated from each other.
Also, the battery described in claim 2 of the present invention uses a boron nitride nanotube as an insulating layer.
Further, according to the battery described in claim 3 of the present invention, first and second electrodes connected respectively to the single-layer carbon nanotubes having different diameters. The single-layer carbon nanotubes are insulated from each other via the boron nitride nanotube.
Additionally, according to the battery described in claim 4 of the present invention, the carbon nanotubes and the boron nitride nanotube have a common tube axis.
Furthermore, according to the battery described in claim 5 of the present invention, after discharging, charging is performed when a potential difference is made between the first and second electrodes.